ARC: Remove b_bufcnt/b_ebufcnt from ARC headers

In most cases we do not care about exact number of buffers linked
to the header, we just need to know if it is zero, non-zero or one.
That can easily be checked just looking on b_buf pointer or in some
cases derefencing it.

b_ebufcnt is read only once, and in that case we already traverse
the list as part of arc_buf_remove(), so second traverse should not
be expensive.

This reduces L1 ARC header size by 8 bytes and full crypto header by
16 bytes, down to 176 and 232 bytes on FreeBSD respectively.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #15350

include/sys/arc_impl.h

@@ -159,8 +159,6 @@ struct arc_write_callback {
  * these two allocation states.
  */
 typedef struct l1arc_buf_hdr {
-	uint8_t			b_byteswap;
-
 	/* protected by arc state mutex */
 	arc_state_t		*b_state;
 	multilist_node_t	b_arc_node;
@@ -171,7 +169,7 @@ typedef struct l1arc_buf_hdr {
 	uint32_t		b_mru_ghost_hits;
 	uint32_t		b_mfu_hits;
 	uint32_t		b_mfu_ghost_hits;
-	uint32_t		b_bufcnt;
+	uint8_t			b_byteswap;
 	arc_buf_t		*b_buf;
 
 	/* self protecting */
@@ -434,12 +432,12 @@ typedef struct l2arc_dev {
  */
 typedef struct arc_buf_hdr_crypt {
 	abd_t			*b_rabd;	/* raw encrypted data */
-	dmu_object_type_t	b_ot;		/* object type */
-	uint32_t		b_ebufcnt;	/* count of encrypted buffers */
 
 	/* dsobj for looking up encryption key for l2arc encryption */
 	uint64_t		b_dsobj;
 
+	dmu_object_type_t	b_ot;		/* object type */
+
 	/* encryption parameters */
 	uint8_t			b_salt[ZIO_DATA_SALT_LEN];
 	uint8_t			b_iv[ZIO_DATA_IV_LEN];